535 research outputs found
INTEGRAL study of temporal properties of bright flares in Supergiant Fast X-ray Transients
We have characterized the typical temporal behaviour of the bright X-ray
flares detected from the three Supergiant Fast X-ray Transients showing the
most extreme transient behaviour (XTEJ1739-302, IGRJ17544-2619,
SAXJ1818.6-1703). We focus here on the cumulative distributions of the
waiting-time (time interval between two consecutive X-ray flares), and the
duration of the hard X-ray activity (duration of the brightest phase of an SFXT
outburst), as observed by INTEGRAL/IBIS in the energy band 17-50 keV. Adopting
the cumulative distribution of waiting-times, it is possible to identify the
typical timescale that clearly separates different outbursts, each composed by
several single flares at ks timescale. This allowed us to measure the duration
of the brightest phase of the outbursts from these three targets, finding that
they show heavy-tailed cumulative distributions. We observe a correlation
between the total energy emitted during SFXT outbursts and the time interval
covered by the outbursts (defined as the elapsed time between the first and the
last flare belonging to the same outburst as observed by INTEGRAL). We show
that temporal properties of flares and outbursts of the sources, which share
common properties regardless different orbital parameters, can be interpreted
in the model of magnetized stellar winds with fractal structure from the
OB-supergiant stars.Comment: 10 pages, 8 figures, 1 table. Accepted for publication in MNRAS
(Accepted 2016 January 26. Received 2016 January 25 ; in original form 2015
December 15
The longest observation of a low intensity state from a Supergiant Fast X-ray Transient: Suzaku observes IGRJ08408-4503
We report here on the longest deep X-ray observation of a SFXT outside
outburst, with an average luminosity level of 1E33 erg/s (assuming 3 kpc
distance). This observation was performed with Suzaku in December 2009 and was
targeted on IGRJ08408-4503, with a net exposure with the X-ray imaging
spectrometer (XIS, 0.4-10 keV) and the hard X-ray detector (HXD, 15-100 keV) of
67.4 ks and 64.7 ks, respectively, spanning about three days. The source was
caught in a low intensity state characterized by an initially average X-ray
luminosity level of 4E32 erg/s (0.5-10 keV) during the first 120 ks, followed
by two long flares (about 45 ks each) peaking at a flux a factor of about 3
higher than the initial pre-flare emission. Both XIS spectra (initial emission
and the two subsequent long flares) can be fitted with a double component
spectrum, with a soft thermal plasma model together with a power law,
differently absorbed. The spectral characteristics suggest that the source is
accreting matter even at this very low intensity level. From the HXD
observation we place an upper limit of 6E33 erg/s (15-40 keV; 3 kpc distance)
to the hard X-ray emission, which is the most stringent constrain to the hard
X-ray emission during a low intensity state in a SFXT, to date. The timescale
observed for the two low intensity long flares is indicative of an orbital
separation of the order of 1E13 cm in IGRJ08408-4503.Comment: Accepted for publication in MNRAS. Accepted 2010 July 6. Received
2010 July 6; in original form 2010 June 9. The paper contains 5 figures and 3
table
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